The invention is based in part on membrane technology such as that taught by U.S. Pat. Nos. 4,209,299 (Carlson '299) and 5,132,094 (Godec et al. '094), which arc incorporated herein by reference. It is not practical, however, sensitively to detect and measure urea or related compounds in ultrapure water by means of urease conversion to CO2 and NH3 using the membrane-based method of Carlson '299 because any presence of trimethylamine, another common contaminate of ultrapure water, would interfere. U.S. Pat. No. 3,915,804 (Messing '804), which is also incorporated herein by reference, teaches that decomposition of urea in water with urease will produce a change in conductivity of the water. This method, however, is complicated by ionic contamination by other ionic extractables from urease, urease substrate or support, and/or the apparatus. Any presence of these ionic contaminations will compromise correct determination of urea concentration, especially when the urea concentration is very low or where very accurate determinations of urea concentration are required.
U.S. Pat. No. 3,926,734 (Gray et al. '734), which is also incorporated herein by reference, uses a bed of immobilized urease to hydrolyze urea to ammonium ion. Following adjustment of the pH to 11 to form ammonia, the latter is selectively passed through a membrane and pH is measured. This method, however, will also measure volatile amine compounds, such as trimethylamine, that are found in ultrapure water, and therefore produces an incorrect measure of urea.
U.S. Pat. Nos. 4,153,513 (Edelmann et al. '513) and 4,277,560 (Gray et al. '560), which are also incorporated herein by reference, teach methods and apparatus that involve injection of an unknown sample into a flowing buffer stream or other solution. This method will dilute urea concentration in the sample to a much lower concentration thereby making very low level measurements even more difficult or impossible (e.g., if the concentration is lowered below the instrument detection limit).
U.S. Pat. No. 4,476,005 (Tokinaga et al. '005), which is also incorporated herein by reference, describes an electrode consisting of a special urease-immobilized membrane and a selective ammonium ion electrode. The membrane has added amino groups to increase the permeation rate of ammonium ion. This electrode is an improvement over one described by G. G. Guilbault in Handbook of Enzymatic Method of Analysis, (Marcel Dekker, New York 1976) and is faster and more sensitive. However, both of these methods and apparatus are not sensitive enough for measurement of urea in trace concentrations in ultrapure water: Additionally, both of these electrodes will respond to other common contaminates such as trimethylamine.
U.S. Pat. No. 5,133,937 (Frackleton et al. '937), which is also incorporated herein by reference, teaches design of an apparatus that has a thermally controlled chamber for urease or other enzyme. The Frackleton et al. '937 patent, however, does not teach measurement of CO2 to determine concentration of urea.
U.S. Pat. No. 5,556,760 (Nakamura et al. '760), which is also incorporated herein by reference, teaches a method and apparatus for measurement of urease enzyme on a surface of a solid. Urea is added to a chamber that is formed with one wall being the solid surface, and pH is measured to determine an amount of urease. This invention, however, does not measure CO2 produced by urease hydrolysis of urea to CO2 and NH3.
Accordingly, there is no known prior art which teaches or suggests an online sensitive analyzer and methods for detecting urea (or other related compounds which produce members of the carbon dioxide group upon hydrolysis) in ultrapure water or use of such an apparatus to prevent contamination of acid-catalyzed photoresists and of a fabrication facility (fab) for preparing such photoresists. Apparatus and methods or entities invention overcome in whole or in part limitations and deficiencies of the prior art.